1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, to a method of forming an isolation layer of a semiconductor device.
2. Discussion of the Related Art
A conventional method of forming an isolation layer of a semiconductor device will be discussed with reference to the attached drawings. FIGS. 1a to 1f are cross-sectional views showing process steps of a conventional method of forming an isolation layer of a semiconductor device.
Referring to FIGS. 1a-1f, the conventional method utilizes a LOCOS (local oxidation of silicon) process using nitride sidewalls to form an isolation layer. As shown in FIG. 1a, on a semiconductor substrate 1, a pad oxide layer 2 is formed on which a first nitride layer 3 is deposited.
Referring to FIG. 1b, a photoresist layer is coated on the first nitride layer 3 and patterned so that the photoresist layer is removed over only a device isolation region. Using the remaining photoresist layer as a mask, the exposed first nitride layer 3 and the pad oxide layer 2 thereunder are etched. Then, the remaining photoresist layer used as the mask is removed.
Referring to FIG. 1c, on the exposed surface of the substrate 1, there is formed a thin oxide layer 4a. Then, a second nitride layer is deposited on the entire surface including the thin oxide layer 4a and is subjected to etch back, thereby forming nitride sidewalls 5 on the sides of the first nitride layer 3.
Referring to FIG. 1d, using the first nitride layer 3 and the nitride sidewalls 5 as masks, the exposed thin oxide layer 4a and the substrate 1 thereunder are etched.
As shown in FIG. 1e, the exposed surface of the substrate 1 is oxidized to form another thin oxide layer 4b. Then, a field ion implantation process is carried out. At this time, the thin oxide layer 4b is used to release stress generated in the substrate 1 during the field ion implantation process. Subsequently, a field oxidation process is performed with the first nitride layer 3 and the nitride sidewalls 5 serving as masks, thereby forming a field oxide layer 6 as shown in FIG. 1f. Next, the first nitride layer 3, the nitride sidewalls 5, and the pad oxide layer 2 are removed.
In the process of forming the device isolation layer according to the above method, a LOCOS process is carried out by using the nitride sidewalls 5. To prevent an abnormal growth of the field oxide layer 6, e.g. bird's beak, the substrate 1 in the field oxidation region is etched.
In the same etching process as discussed above, a trench is formed in the substrate 1 and a field oxidation process is carried out. As shown in FIG. 1f, step coverage occurs between the periphery portion of the field oxide layer 6 and the substrate 1. Therefore, the active region defined by the first field mask can prevent the field oxide layer from growing abnormally.
In a conventional method, a defined active region can prevent abnormal growth of a field oxide layer in an isolation layer. However, since the periphery portion of the field oxide layer is formed to be in a lower position than the position of the surface of the substrate, a gate line crossing the periphery portion of the field oxide layer causes step coverage in the following step, thereby degrading device characteristics.
The generation of step coverage affects device planarization and thus affects metal wiring, as well. Moreover, since the periphery portion of the field oxide layer is formed in a lower position than that of the surface of the substrate, the periphery portion causes a reduction in process tolerance, thereby causing difficulties in processing. Moreover, since the nitride sidewalls extend up to the sides of the substrate in the field oxidation process, the substrate receives a large amount of stress, thereby degrading electric characteristics of the device.